1. Field of the Invention
The present invention relates to a vertical deflection circuit to be used in a color picture tube apparatus of a television or a display monitor.
2. Description of the Prior Art
In in-line color picture tubes used in televisions and display monitors, when there is a misalignment between the central axis of the vertical deflection magnetic field and the central axis of the color picture tube, or there is a rotational misalignment between the vertical deflection magnetic field and the color picture tube, mis-convergence occurs separately at the upper and lower portions of the color picture tube screen (fluorescent screen).
As means for separately correcting these kinds of mis-convergences at the upper portion and the lower portion of the color picture tube's screen, Publication of Unexamined Japanese Publication No. Hei 8-102270 discloses two auxiliary coils that are provided at a rear portion, i.e. on the electron gun side, of a deflection yoke. Those auxiliary coils are supplied with correction current by a convergence correction circuit that uses a diode bridge circuit.
FIG. 4 shows a conventional vertical deflection yoke circuit, and the curve (a) in FIG. 5 shows the impedance characteristics of this conventional vertical deflection yoke.
As shown in FIG. 4, the vertical deflection yoke comprises vertical deflection coils 60 installed in a color picture tube 70, a convergence correction circuit 2 that uses a diode switch, and vertical coma aberration correction coils 209a, 209b, 209c, 209d, 210a, and 210b. The convergence correction circuit 2 comprises a resistor 201 and a resistor 202, which are connected in series. Connected in parallel thereto is a series circuit including a diode 203, a variable resistor 205, and a diode 204 disposed in the same direction as the diode 203. Further, a series circuit, including a diode 206 which is disposed in the direction reverse to the diode 203, a variable resistor 208 and a diode 207 disposed in the same direction as the diode 206, is connected in parallel to the series circuit including the resistors 201 and 202. The convergence correction circuit 2 is a part of a vertical deflection circuit.
However, if such a convergence correction circuit is added to correct mis-convergence the impedance characteristics of the vertical deflection yoke 3 change abruptly near the center of the vertical deflection period, or in other words, near a deflection current value of 0 A, as shown by curve (a) in the center of FIG. 5. This is due to an impedance change of the convergence correction circuit 2 in response to the voltage-current characteristics of the diodes 203, 204, 206 and 207 in the convergence correction circuit 2. For this reason, the vertical deflection voltage that is supplied to the vertical deflection yoke cannot follow an impedance change of the vertical deflection yoke with respect to the deflection current, and the rate of the vertical deflection decreases near the vertical center of the color picture tube screen. Therefore, the vertical density of scanning lines near the vertical center of the screen increases, causing the problem that a white horizontal belt-like area appears on the screen.
This type of white belt-like area also has appeared with convergence correction circuits other than that explained in this prior art example, for example, in circuits where a correction means using the switching characteristics of a diode were added to the vertical deflection circuit.